The present invention relates to an electromagnetic sound producing device which is mounted on a printed circuit of an electronic equipment by the surface-mount technology.
In recent years, various electronic equipments have become smaller in size and lighter in weight. Not only operational elements, memories and connectors, but other electronic devices have come to be directly mounted on a printed circuit as well. An electromagnetic sound producing device which is one type of a compact buzzer, is now generally directly mounted on the surface of the printed circuit by the surface-mount technology. Moreover, in order to realize a printed circuit of high-density and multilayer, it is necessary that the mounting position of the device can be arbitrarily selected.
On the other hand, the sound producing device has a sound emanating hole which is formed in a wall of a casing opposite an armature thereof. In order to generate a large sound even at a low frequency, it is necessary to form a release opening in the opposite wall of the casing at the other side of the armature so that back pressure caused by the vibration of the armature is released. Due to the restrictions imposed on the structure of a die for manufacturing the casing, the release opening is formed in a base which forms a bottom of the casing of the sound producing device, attached to the printed circuit board.
More particularly, in the die comprising a lower die and upper die, the upper die is upwardly moved when taking out a moulding. In order to form the opposite release opening, a core pin must be inserted in the moulding material. However, in order to form the opening in the vertical side wall, the core pin must be inserted in the horizontal direction. To raise the upper die, the core pin must be retracted before the raising. Such a die causes the construction thereof to be very complicated. Therefore, there has not been provided such a complicated die.
FIGS. 5 and 6 show a conventional surface-mount sound producing device manufactured by a conventional die. The surface-mount sound reproducing device is mounted on a printed circuit 50. The sound producing device comprises a plastic lower casing 51 directly mounted on the upper surface of the printed circuit 50 and an upper plastic casing 52 mounted on the lower casing 51. The lower casing 51 comprises a substantially square base 51a and four peripheral walls 51c attached to the base 51a. The four corners of the peripheral wall 51c are chamfered, as shown in FIG. 6.
Referring to FIG. 6, securely mounted on the underside of the base 51a are externally connecting terminals 53 and 54, each having a square shape, embedded therein at two adjacent corners thereof, and metal plates 55 and 56 embedded at other corners. The externally connecting terminals 53 and 54 are provided to connect a coil of the sound producing device with a circuit formed on the printed circuit 50, and plates 55 and 56 are provided for ensuring the adhesion of the device to the printed circuit. When mounting the sound producing device on the printed circuit 50, the two externally connecting terminals 53 and 54 are adhered to, and hence electrically connected with the terminals of the printed circuit 50 by means such as soldering. The plates 55 and 56 are likewisely adhered to the printed circuit 50.
In order to accurately position the upper casing 52 on the lower casing 51, there are provided four guide blocks 68 projecting vertically upward from the lower casing 51 at the four corners as shown in FIGS. 7 and 8. The guide block 68 is disposed inside the outer peripheral wall 51c so as to form a guide face 68a for the upper casing 52.
Referring back to FIG. 5, in the lower casing 51, there is securely mounted a magnetic yoke 57 having a center pole 58. A cylindrical armature supporting member 60 made of brass is securely mounted on the yoke 57 with adhesive. A coil 66 is mounted on the yoke 57, coaxial with the center pole 58. By mounting the coil, both ends of the coil are electrically connected to the respective externally connecting terminals 53 and 54 through inside terminals 76 and 77 (FIG. 7) in the base 51a of the lower casing 51. A cylindrical permanent magnet 65 which is vertically magnetized is disposed on the yoke 57 so as to surround the coil 66.
An armature 61 having a magnetic pole piece 62 secured on the surface at the center thereof is mounted on the upper edge of the supporting member 60, and attracted by magnetic force described hereinafter, thereby defining a lower chamber 63 and an upper chamber 64 in the space inside the lower and upper casings 51 and 52. The yoke 57, coil 66 and the permanent magnet 65 compose an actuator for driving the armature 61. The armature 61 is normally attracted to the supporting member 60 by the magnetomotive force of the permanent magnet 65, and held by an inside wall of an upward projection 68b (FIG. 8) of the guide block 68.
The upper casing 52 has a shape similar to that of the lower casing 51 and has four peripheral side walls 52a and a square cover plate 52b. The four corners of the upper casing 52 are chamfered to conform to the periphery of the lower casing 51. A plurality of armature stoppers 67 are formed on the underside of the cover plate 52b, forming a small gap between the lower surfaces of stoppers 67 and the armature 61.
The upper casing 52 is mounted on the lower casing 51 so that the guide face 68a of the guide block 68 snugly fits therein. The lower and upper casings 51 and 52 are further adhered to each other by ultrasonic welding.
The sound producing device is further provided with a sound emanating hole 70 formed in the cover plate 52b of the upper casing 52 at the center thereof. A plurality of sound passages 71 are formed in a housing 72 confronting the sound emanating hole 70. The base 51a and the yoke 57 have openings 73 and 74, respectively, each coinciding with each other. A release hole 75 is further formed in the printed circuit 50 confronting the opening 74 of the casing 51, thereby communicating the lower chamber 63 with the ambience.
In operation, a sound producing device driving signal produced in an electronic circuit (not shown) formed on the printed circuit 50, is applied to the coil 66 of the actuator of the sound producing device through the terminal of the circuit, the externally connecting terminals 53 and 54, and the inside terminals 76 and 77. The coil 66 is hence excited, and in cooperation with the permanent magnet 65, vibrates the armature 61, thereby generating a sound. The sound is emanated out of the housing 72 through the upper chamber 64, sound emanating hole 70 and the sound passages 71. The back pressure which is caused by vibration of the armature 61 is released through an air passage comprising the lower chamber 63, openings 73 and 74 and the release hole 75. Without such an air passage, the back pressure cannot be released, thereby restraining the vibration of the armature 61. As a result, the volume of the generated sound becomes smaller, particularly in a low frequency range. Thus the air passage is indispensable.
In the above described conventional sound producing device, because of the structure of the die for moulding the plastic casing, the opening 74 must be formed in the base 51a of the lower casing 51. Accordingly, it becomes necessary to perforate the release hole 75 in the printed circuit 50 at the position corresponding to the opening 74. This restricts the choice in designing the printed circuit, and hence, becomes an obstacle in producing a high-density printed circuit which is multiple layered, compact, light in weight, and also inexpensive. However, the problem of the die belongs to a technical field other than the electromagnetic sound producing device. Therefore, it will be better to consider the problem of the die, separating from the sound producing device itself.
On the other hand, if the sound emanating hole 70 is formed in the cover plate 52b as in the conventional device of FIG. 5, the sound passages 71 must be formed to directly confront the hole 70 in order that the sound is satisfactorily emanated, However such an inevitable condition restricts the design choice of the housing 72.